Fluorinated powder coatings having regulable gloss

Abstract: Pigmented powder coating products based on low melting point fluorine-based terpolymers, preferably on those made from a majority of vinylidene fluoride copolymerised with tetrafluoroethylene and hexafluoropropene, are capable of providing regulable gloss coatings. The process for preparing coatings from the powders is essentially characterised by the selection of the temperature by which the desired gloss can be obtained. Low to high gloss can be obtained, and the mechanical properties are excellent.

Description: The present invention relates to fluorinated powder coating products, based on certain vinylidene fluoride terpolymers (hereinafter referred to as VdFT), for preparing fluorinated coatings having regulable gloss and to a process for coating a substrate with a pigmented fluorinated coating having regulable gloss. In particular, the present invention relates to the use of certain vinylidene fluoride terpolymers in pigmented powder coating products, for obtaining fluorinated protective coatings having regulable gloss. Coatings based on vinylidene fluoride homopolymers (hereinafter PVdF) are known to be very useful as protective coatings for a variety of substrates due to the good chemical and weathering resistance and to the thermal stability of PVdF. The general known technique used for preparing them is to prepare a dispersion of PVdF in a suitable solvent for application by known means onto the required substrate which is thereafter subjected to a heat treatment. The solvent used is generally known in the prior art as "latent solvent", and it is described therein as an organic solvent which has no significant action on PVdF at room temperature but which at an elevated temperature exerts a sufficient solvent action. However, although the known systems may give good results, environmental protection laws throughout the world, but mainly in Europe and in the U.S.A., are making it increasingly difficult to handle solvent-based systems. Further, the recovery of the solvent is a costly procedure. Accordingly, there is a need in the art for solventless PVdF-based coatings. GB-2194539-A to Labofina discloses pigmented PVdF-based powder coating products essentially consisting of vinylidene fluoride homopolymer (or copolymer with at most 10 wt% of comonomer units), one or more compatible thermoplastic resin and one or more pigments. It also discloses a process for preparing the products. However, the gloss obtained is not regulable. EP-456018-A to Elf Atochem North America discloses pigmented powder coatings of vinylidene fluoride/hexafluoropropylene copolymers, the resin component comprising 50-90 wt% of said copolymers having a melt viscosity of 1-4 kPoise [100-400 Pa.s] at 100 s -1 and 232°C, said copolymers being characterised by a melting point in the range of about 160°C to about 170°C (which closely approaches the melting point of PVdF), and 50-10 wt% of thermoplastic acrylic resin. The resulting coatings are said to be characterised by improved flexibility, crack resistance and surface smoothness (as opposed to surface roughness or "orange peel" associated with the finish) whilst not requiring the inclusion of a flow-improving agent. No information is reported about the gloss, not to mention gloss regulation, except that the thermoplastic acrylic resin is responsible for gloss durability. US Patent 5,229,460 to Evodex discloses fluoropolymer-based powder coatings which may be based on PVdF or on a copolymer with at least 80 wt% of vinylidene fluoride and up to 20 wt% of at least one other fluorine-based monomer. No information is reported about the gloss, not to mention gloss regulation. There is however a certain demand for very low glosses, which cannot be obtained presently. Accordingly, there is a need for a process for regulating the gloss of fluorinated powder coatings. An object of the present invention is to provide VdFT-based powder coating compositions capable of providing regulable-gloss coatings. Regulable-gloss, as used herein, are coatings whose gloss can be adjusted from values above 40, preferably above 50 and most preferably above 60, down to values below 30, preferably below 20 (all values being measured according to ISO 2813 at an angle of 60°), while still having a good surface smoothness. Another object of the present invention is to provide VdFT-based powder coating compositions capable of providing low-gloss coatings. Low-gloss coatings, as used herein, are coatings having a gloss below 30, preferably below 20 (all values being measured according to ISO 2813 at an angle of 60°), while still having a good surface smoothness. A further object is to provide a process for preparing regulable-gloss VdFT-based coatings. Still another object is to provide a process for preparing low-gloss VdFT-based coatings. Accordingly, the present invention provides a powder coating product, essentially consisting of : (i) a resin component essentially consisting of : (a) from 60 to 90 wt% of one or more fluorine-based terpolymer having a melting temperature below 150°C; (b) from 40 to 10 wt% of one or more compatible resins; and (ii) from 1 to 35 parts by weight of one or more pigments per 100 parts by weight of the resin component. The present invention further provides a process for preparing coatings from the powder coating product of the invention, essentially characterised by a selection of the baking temperature. The low melting point fluorine-based terpolymers used herein are made from three monomers selected from vinylidene fluoride (VdF), tetrafluoroethylene (TFE), chlorotrifluoroethylene, vinyl fluoride, hexafluoropropene (HFP), and CF 3 -CF 2 -CF=CF 2 , and having a melting temperature below about 150°C, preferably below 120°C and most preferably below 100°C. Preferably, the terpolymer is made from a majority of VdF copolymerised with TFE and HFP; most preferred copolymers are prepared from 65 to 85 wt% of VdF monomer units with from 7.5 to 17.5 wt% of each of TFE and HFP. The melt viscosity of the terpolymer should preferably be of from 9,000 to 15,000 dPa.s, most preferably from 11,000 to 13,000 dPa.s, all measured at 125°C. The VdFT is mixed with one or several compatible resins, preferably of the acrylic type. Acrylic resins are known in the art and need not be described here; a description may e.g. be found in FR-2,636,959-A to Atochem (from line 18 of page 3 to line 14 of page 4). As examples of thermosetting acrylic resin, there may in addition be cited those described in US-4,659,768 to DeSoto, particularly under the denominations "Experimental resin" and "control resin". It has however been found that it is advantageous to use a thermoplastic acrylic resin, and most advantageous to use a thermoplastic polymethylmethacrylate (PMMA) resin which is therefore preferred. As a thermoplastic PMMA resin, there may be used a thermoplastic resin obtained by (co)polymerisation of at least 75 wt% of an alkyl methacrylate, the other comonomers being one or several olefinically unsaturated comonomers, preferably of the alkyl (meth)acrylate type. The esters are formed by the reaction of the acrylic or methacrylic acid with suitable alcohols, for example, methyl alcohol, ethyl alcohol, propyl alcohol, butyl alcohol and 2-ethylhexyl alcohol. Generally, the larger the alcohol portion of the ester, the softer and more flexible the resultant resin. Also, generally, the methacrylic esters form harder films than the corresponding acrylic esters. As examples of such resins, there may be cited polymethylmethacrylate, copolymers of methylmethacrylate with ethyl acrylate, butyl methacrylate, isobutyl methacrylate, acrylic acid or methyacrylic acid, and the like. The most preferred PMMA resins are those which exhibit a viscosity of from 7 to 17 P in 40% solution in a 95:5 by weight mixture of toluene and ethylene glycol methyl ether. The weight ratio of the VdFT to the compatible resin may vary widely from 90:10 to 60:40, preferably from 75:25 to 65:35, most preferably of about 70:30. The resin component may additionally contain the usual minor amounts of the usual additives, such as for example an antioxidant, a flow promoting agent, an UV absorber and/or an adhesion promoter. As suggested in US Patent 5,229,460, an antioxidant may if necessary enhance heat stability and provide protection against thermal degradation and discoloration of the coating film due to excessive heat curing temperatures; also, the use of Kynar ADS™ as substitute for a flow promoting agent is disclosed. The use of pigments in the coating composition is preferred. Indeed, if no pigment is used, a clearcoat or varnish can be obtained; however, the resulting coating is unevenly milky, which is undesirable. Further, such clearcoats have a lesser resistance to high temperature, and most important they insufficiently absorb ultraviolet light which will therefore be able to contribute to the degradation of the primer if any. The use of ultrafine titanium dioxide to absorb ultraviolet light is however known in the art. When pigments are used, there may be used any pigment or combination of pigments. The choice of pigments should preferably be made in accordance with what is known in the art for PVdF-based coatings. The amount of pigment used may vary widely, especially according to its hiding power. For example, a white topcoat prepared exclusively with titanium dioxide may require up to about 35 wt% of said pigment. Other pigments with better hiding properties may require lower amounts. The process for preparing the powder coating product essentially comprises the steps of : (i) melt-mixing the VdFT, the compatible resin components and the pigments, (ii) forming pellets, and (iii) grinding the pellets. Melt-mixing is normally carried out by extrusion. Extruding and granulating the resulting mixture can be made by the usual procedures. The operating procedure is easily determined by one skilled in that art. In particular, there may be used a single or double screw extruder. The processing temperature is usually of from 100°C to 130°C. The dimensions of the pellets are not critical parameters; they are usually of about 3 mm diameter and 2 mm length. Grinding the pellets may be done by any suitable means which allows suitable particles to be obtained. Techniques for grinding are known to those skilled in the art and need not be described further. US Patent 5,229,460 contains a discussion of the state of the art of grinding fluoropolymer-based thermoplastic blends and the influence of the cooling step thereon. The powder should comprise particles of size and shape suitable for achieving constant flow through the application equipment, mainly in order to obtain a constant thickness of the coating. It is preferred that the shape of the particles be as spherical as possible and their size as uniform as possible, because the resulting powder has better flowing properties. Regarding the size of the particles, the grinding step is associated with a sieving step for eliminating the largest size particles, i.e. those particles exceeding about three times the desired thickness of the coating. On the other hand, excessively small particles are to be avoided, because they are a hazard for health and because they tend to block the transport lines during application. It is preferred to use a hammer mill wherein a rotating shaft carries hammers which break the pellets on fixed shapes in the casing of the hammer mill and hammer them through a sieving lining at the bottom of the hammer mill. Sieve openings of about 0.2 mm have been found particularly appropriate. The resulting powder may be applied on the substrate by any means suitable for achieving an even distribution of the particles. In particular, there may be used any electrostatic spray application apparatus, whereby the charged particles are sprayed onto the oppositely charged substrate. Alternative techniques are cloud chamber, fluidised bed, and even triboelectric coating, and the like. Such techniques are well known in the art and need not be described further. The VdFT-based powder is preferably applied over a suitable primer coating, e.g. as known in the art as primers for similar fluorinated coatings, including solvent-based, "flash" or powder primers. After the coating has been applied to the substrate, it must be subjected to a heat treatment. The coated substrate is passed into a heated oven wherein the coating is finally baked. An essential feature of the process of the invention is the selection of the temperature, which enables to control the gloss. The temperature to be selected for obtaining a specific gloss (within experimental variations) is easily determined by one skilled in the art, using an oven with a temperature gradient; such material is currently used in the art and need not be described further. This feature is based on the observation that the gloss obtained with the compositions of the invention first increase, then decrease to a low value (as defined herein). Finally, the coating and its substrate can be either slowly cooled in air or quenched in water. In order to further illustrate the invention, the following example is given which is intended not to limit the scope of the invention. This example shows that the desired gloss, from a high gloss to a low gloss, can be obtained by selecting a baking temperature between about 140°C (i.e. about 50°C above the melting temperature) to about 220°C (i.e. well below the highest temperature at which the polymer is stable). It also shows that better mechanical properties can be obtained than with PVdF. Example 1 The following white powder coating composition was prepared : - fluorinated polymer 54.9 pbw - acrylic polymers 24.0 pbw - titanium dioxide 21.1 pbw 100.0 As fluorinated polymer, there was used a terpolymer consisting of about 75 wtX of VdF monomer units and about 12.5 wt% of each of HFP and TFE monomer units (as determined from NMR data by the method of Pianca et al. in Polymer 28, 224, 1987), said polymer having the following properties according to its technical datasheet : - melting point about 90 °C ASTM D-3418 - specific gravity 1.846 g/cm3 ASTM D-792 - refractive index (n D 25 ) 1.384 ASTM D-524 - tensile strength at yield 8 MPa ASTM D-882 - tensile strength at break 15 MPa ASTM D-882 - elongation at break 990 % ASTM D-882 - tensile modulus 130 MPa ASTM D-882 - solution viscosity 20 mPa.s Brookfield LV (20 wt% in MEK at 25°C) #1 spindle @ 30 rpm - stability up to 316 °C The acrylic polymer was essentially a commercially available thermoplastic copolymer of 70 wt% methyl methacrylate monomer and 30 wt% of ethyl acrylate monomer, having a viscosity of about 12 P in 40% solution in a 95:5 by weight mixture of toluene and ethylene glycol methyl ether, containing 3 wt% (relative to the total acrylic polymers) of a low molecular weight copolymer of 30 wt% of ethyl acrylate monomer and 70 wt% of 2-ethylhexyl acrylate monomer, having a viscosity of about 1.06 Pa.s at 98.9°C. The resulting mixtures were extruded under the following conditions, giving pellets of 3 mm diameter and about 2 mm length : twin screw extruder screw rotation : 200 rpm load : 85% temperature profile : 100°C at hopper exit, rising to 130°C in the middle of the screw, then 115°C up to the end of the screw temperature of the material at the exit : 115°C. The pellets were cooled in liquid nitrogen down to about -150°C, then grinded at a temperature of about -100°C in a hammer mill and sieved to eliminate particles larger than about 150 µm. In the hammer mill, a rotation shaft carries hammers which break the pellets on fixed shapes in the casing of the hammer mill and hammer them through a sieving lining at the bottom of the hammer mill. The particle size distribution was measured : 99% of the particles had a size below 90 µm, and 40% below 32 µm. Only 5% had a size below 15 µm. The resulting powders were applied by an electrostatic spray process on a 1 mm thick rolled steel plate previously covered by a 50 µm layer of an epoxy primer. The epoxy primer had been prepared and applied according to example 1 of EP-404752-A to the Applicants. Two coated substrates were then heated 20 minutes in a temperature-gradient oven, giving 80 µm thick top coatings. The following results were obtained in the two separate determinations: Temperature (°C) 121 126 131 138 144 150 157 164 171 179 Gloss (60° ISO-2813) 49 56 59 62 60 61 55 51 44 30 Temperature (°C) 149 158 168 177 188 199 210 222 235 249 Gloss (60° ISO-2813) 59 50 41 26 20 18 18 16 16 16 A further coated substrate was baked at 200°C during 10 minutes, and the following properties were measured after cooling : - direct impact >100 kg.cm ASTM D-2794 - reverse impact >100 kg.cm ASTM D-2794 - flexibility (conical mandrel) 0 mm ISO 6860 - flexibility (Erichsen) 7.7 mm ISO 1510 - adherence 0 GT ISO 2409

Claims: Powder coating product, essentially consisting of : (i) a resin component essentially consisting of : (a) from 60 to 90 wt% of one or more fluorine-based terpolymers having a melting temperature below 150°C; (b) from 40 to 10 wt% of one or more compatible resins; and (ii) from 1 to 35 parts by weight of one or more pigments per 100 parts by weight of the resin component. Powder coating product according to claim 1, wherein the terpolymers have a melting temperature below 120°C, most preferably below 100°C. Powder coating product according to either of claims 1 and 2, wherein the terpolymers are made from three monomers selected from vinylidene fluoride (VdF), tetrafluoroethylene (TFE), chlorotrifluoroethylene, vinyl fluoride, hexafluoropropene (HFP), and CF 3 -CF 2 -CF=CF 2 . Powder coating product according to any one of claims 1 to 3, wherein the terpolymers are prepared from 65 to 85 wt% of VdF monomer units with from 7.5 to 17.5 wt% of each of TFE and HFP. Powder coating product according to any one of claims 1 to 4, wherein the terpolymers have a melt viscosity of from 9,000 to 15,000 dPa.s, most preferably from 11,000 to 13,000 dPa.s, all measured at 125°C. Powder coating product according to any one of claims 1 to 5, wherein the compatible resin is thermoplastic polymethylmethacrylate. Powder coating product according to any one of claims 1 to 5, wherein the weight ratio of the terpolymer to the compatible resin is of about 70:30. Process for preparing coatings from the powder coating product according to any one of claims 1 to 7, essentially characterised by a selection of the baking temperature. Use of the powder coating product according to any one of claims 1 to 7 to produce regulable-gloss coatings. Use of the powder coating product according to any one of claims 1 to 7 to produce low-gloss coatings.

Inventors: GILLARD MICHEL, RIJKSE LUDWIG KARL, VERWEY EDWIN, GILLARD, MICHEL, RIJKSE, LUDWIG KARL, VERWEY, EDWIN
Applicants: FINA RESEARCH, FINA RESEARCH S.A.
Application year: 1994
Main classification: C09D5
IPC codes: C08L27, C08L33, C09D5, C09D127, C09D133